An eating disorder is a mental and physical illness that is characterized by a preoccupation with food and weight. This condition is often so serious that the person with the eating disorder is able to focus on very few things other than the food they eat, how much they weigh, and how they appear to others. For example, a person with an eating disorder may eat extremely small amounts of food or none at all. They may also spend hours looking at themselves in the mirror. A person with another type of eating disorder may overeat or eat in secret.